Prefabricated plastic-sheathed pipes are commonly used in district heating systems. When a district heating pipeline is laid, prefabricated pipe lengths are connected to each other, whereafter the joint area is sealed so as to provide a permanent, insulated, watertight joint that can withstand the operating conditions and loads.
For small and medium diameter pipes, normally a standard sleeve joint is used for sealing the joint area. The sleeve bridges the two pipe sheath ends involved while overlapping said sheath ends. Mostly, the sleeve ends are sealed to the pipe sheaths by means of shrink-on gaiters.
However, when large diameter pipes and high mechanical stresses are involved, it has proved that the standard sleeve joint is not adequate, because the heat necessary for the shrinking process is difficult to apply in a proper way and the joints obtained have a tendency to crack and give leaks, thus leading to a damaged pipeline.
Instead, when large diameter pipes are involved, V butt welding of a joint sleeve has been used, the joint sleeve having the same outer diameter as the pipe sheath.
Also, a so-called "Swedish Joint" has been used. This is a sleeve joint wherein the overlapping sleeve ends are sealed to the underlying pipe sheath ends by means of intermediate heat welded material including electrical heating means and pressurizing means.
It has been proposed to seal the ends of the sleeve of a sleeve joint merely by melting together said ends and the outer layer of the adjoining pipe sheaths, the sleeve having an inner diameter corresponding to the outer diameter of the pipe sheath. The melting heat is supplied via a heating ring applied around the joint area. This method have proved not to be successful.
It has also been proposed, in general terms, that the sealing of the overlapping sleeve ends of a sleeve joint could be made by fillet welds, such welds then replacing the shrinkon gaiters. However, there have been no practical suggestions as to how the fillet welds should be made and no practical experiences of on-site use have been discussed.